Image combiner eyepieces are a useful part of current technology. Such a device is typically used to superimpose relevant information (e.g. target or road and rail network information) over the field of view available to an observer. In this way, the observer can assimilate information from several sources without diverting his attention from the real scene in his line of vision.
It has been difficult, however, to design a light, compact eyepiece which accurately maps the enhancement information onto the outside view. Two related problems complicate the design process. The first problem is the need to project the enhancement image to a distance of virtual infinity, the farthest distance to which the human eye can specifically focus. The second problem is the need to provide a "viewing area" of an acceptable size.
The human eye determines the relative angular location of objects by the orientation of the light wave that reaches the eye from that object. For close objects, the orientation of the light wave from an object changes as one moves one's eyes relative to that object. For objects up to about 8 feet away the two eyes determine the angular difference between the light reaching each of them. The brain uses this information to form a stereoscopic distance estimate. For distant objects, by the time the light reaches the observer, it has spread out spherically and the sphere shell portion that reaches a particular distant observer is essentially a plane wave with parallel rays.
As a result, a distant scene doesn't change appearance as one moves a short distance. The relative position of a close object in front of a distant scene, however, does change with such movement. This is one reason why the enhancement imagery, which by necessity is generated quite close to the viewer, must be projected to virtual infinity. Otherwise, the position of the enhancement imagery would change relative to the outside scene whenever the viewer moved his head. As the physical features depicted in the enhancement imagery will almost always be located at virtual infinity, this position change would be unrealistic and inaccurate. Another reason for projecting the enhancement image to infinity is so that the viewer is not required to refocus his eyes to see it clearly.
In order to make a close object appear at virtual infinity, the rays of light from that object are caused to be parallel over a particular region. Light that has been directed in this manner is referred to as "collimated" light.
One problem with collimated light is that, unlike light that is reflected from a random object, collimated light does not propagate outwardly in all directions. Instead, it travels in a beam, strictly in one direction. As a result, in order to receive collimated light from a particular source, the eye must be at the correct location. Otherwise, the beam of collimated light will be completely missed and no light source will appear to the observer.
Image combiner eyepieces, such as the present invention, are designed to have a specific location where a viewer may place his eyes. This location is known as the "viewing area". The size of the viewing area is important because it determines whether a viewer can place his eyes at slightly different locations and still obtain a clear image. For a very small viewing area, any slight movement on the part of the viewer would cause the image to disappear. The width and height of the beam of collimated light from any particular point source determines the size of the viewing area of an eyepiece.
Generally, in the design of eyepieces for use with enhanced imagery, a particular viewing area size is specified in order to allow the user to shift eye location with respect to the eyepiece without immediately losing sight of the enhancement imagery. Also, a large enough viewing area size accommodates different users whose facial structures and viewing postures may cause them to use the eyepiece with their eyes positioned slightly differently relative to the viewing area.
It should be noted that a number of other approaches to the problem of displaying information at virtual infinity have been disclosed in prior art patents. All of the relevant patents cited here, however, are from the related field of head up displays. Although some of the problems encountered in designing head up displays are similar to those encountered in the design of image combining eyepieces, there are some noticeable differences. A review of the relevant differences will help to explain the different function of some apparently similar structures found in both the prior art references and in the present invention.
As in the field of the present invention, the object of a head up display is to superimpose information and/or imagery over the real field of view of the observer. In a head-up display this is achieved by collimating the light emitted from the information or image source and then projecting it toward a partially reflective mirror oriented at about a 45.degree. angle with respect to the observer. (A partially reflective mirror reflects a portion of the light impinging on one side and also transmits a portion of the light impinging on the other side.) In the case of the head up display application, a portion of the light from an image source is reflected to the observer while a portion of the light from the real world is transmitted through the partially reflective mirror to the observer. The observer, however, is located at a much greater distance (measured in feet) from the display screen than is the case for the user of an eyepiece (measured in inches). As a result, the mirror onto which the image is projected must be much larger to convey the same angular field of view. The geometry of the inside of the vehicle in which the head-up display is used sometimes may make it difficult to position a large mirror supporting the necessary field of view. To solve this problem, multiple partially reflective combining mirrors may be used to allow multiple viewing geometries of the collimated image source thereby allowing to the observer a larger angular field of view.
In addition, the collimating mechanism for a head-up display may be considerably larger than the same mechanism for an eyepiece. This is because, an eyepiece must be light and compact in order to be easily maneuvered to the eyes of the viewer and then easily removed when no longer needed. As the collimating mechanism must be included in the eyepiece, it must also be small and light. The collimating mechanism for a head up display, on the other hand, can be a stationary, built in item, which need not be moved to be used. As a result, this mechanism may be considerably larger than the comparable item for an eyepiece. With a large collimator, it is possible to get a wide beam of collimated light for every point of light emitted by the image generator. Therefore, it is usually not necessary to further widen the collimated beam in the image combining mechanism.
Because of the different nature of the problems facing the designer of an eyepiece in contrast to those facing the designer of a head up display, a device used for one purpose in a head-up display would not suggest that a similar structure be used for a completely different purpose in an eyepiece.
The prior art references that include multiple combiner screens are, Ellis, U.S. Pat. No. 4,099,841; Ellis, U.S. Pat. No. 4,611,877; Wood, U.S. Pat. No. 4,655,540; and Suvada, U.S. Pat. No. 5,200,844, which is assigned to the assignee of the present invention. Also, the copending application of Suvada, Ser. No. 07/887,546, (assigned to the assignee of the present invention) also includes a reference to such a feature. In all of these cases, the multiple combiner screens are used to increase the field of view or enhance holographic effects rather than to expand the viewing area. In Suvada '844 the different screens have color specific reflective characteristics to enhance the color display.
One may gain a general appreciation for the different sorts of problems faced by head-up display designers by examining the prior art patents. Starting with the earliest, Ellis '841 discloses an image combiner mechanism including three partial mirrors with varying degrees of reflectivity and transmissiveness to achieve an evenly bright display of enhancement and real world imagery from each of the three partial mirrors. This creates a large field of view for the observer.
Ellis '841 also briefly discloses a mechanism for the collimation of the light from the imagery/information source, which, in that disclosure was a cathode ray tube. Two sets of lenses, separated by approximately 1 foot, collimate the light. A bend may be introduced between the two sets of lenses with a mirror reflecting the light rays toward the second set. One may note that a 1 foot distance to permit a more complete collimation is not available in an eyepiece.
Ellis '877 shows an improved mechanism for light collimation. A wedge shaped piece of glass is added between the first and second sets of lenses. This bends the light more efficiently than the mirror disclosed in the previous Ellis patent.
Raber, U.S. Pat. No. 4,729,634, uses first and second cooperative and converging mirror elements to produce a collimated beam from a CRT. This reference combines the lens and mirror functions which are separate in the Ellis patents into two curved, one way mirrors. The combination of elements allows a more compact design and prevents destructive internal reflections.
Ferrer, U.S. Pat. No. 4,799,765 permits the projection of images from two sources onto two different display panels. The primary imagery source is reflected from a mirror which will selectively transmit light of a particular frequency. This imagery is then displayed on a curved partially reflective mirror directly in front of the pilot. The secondary imagery (generally flight information) is emitted in the form of light of the frequency passed by the mirror. This light passes through the mirror and is displayed on a screen which is in front of and down from the user's eye location.
Banbury, et al., U.S. Pat. No. 4,927,234 discloses a single screen head up display. A double fold is introduced into the optical axis in order to reduce the size of the device. There is no structure or suggestion, however, of a double partially reflective mirror to increase the vertical extent of the collimated beam as we find in the present invention.
Wood et al., U.S. Pat. No. 4,655,540, and Suvada, U.S. Pat. No. 5,200,844, both use multiple display screens to achieve holographic effects. Wood '540 uses a double display screen to overcome the problems imposed by the duality between the spectral bandwidth and angular bandwidth of a hologram. By using two screens a smaller range of viewing angles is imposed on each and the spectral bandwidth may be increased.
Suvada, U.S. Pat. No. 5,200,844, discloses three different display screens, each one tuned to a different wavelength of light. This enhances the color intensity of a color display. There is no suggestion in this reference, however, to a collimated beam of greater vertical extent.
Suvada, copending application Ser. No. 07/887,546, discloses a prism arrangement for redirecting collimated light in a head-up display. This is more efficient than performing the same function by means of a mirror. Although more than one display screen mirror may be used in this disclosure, this is again directed toward increasing the users field of view rather than his viewing area.
One prior art arrangement, which has been utilized in various helmet mounted display applications to both map the enhancement imagery over the outside scene and to project it to infinity is to place a partially reflective mirror at approximately a 45.degree. angle to the line of sight of the viewer. A partially reflective mirror is a mirror that reflects a portion of the light impinging on one surface and also allows a portion of the light impinging on its other surface to pass through it. The enhancement image is then transmitted through the partially reflective mirror from the image source at the top to a spherical mirror located at the bottom of the device where it is collimated and reflected back toward the partially reflective mirror. Part of the collimated light reflected from the spherical mirror is reflected from the partially reflective mirror to the observer's eye. This light is also combined with the imagery from outside the vehicle which enters the transparent far end of the eyepiece and is transmitted through the partially reflective mirror to the observers's eye.
A specified viewing area size controls the design of the eyepiece. The required viewing area size dictates the distance at which the spherical mirror must be placed from the enhancement imagery source to effect a wide enough collimated beam. This determines the dimensions of the eyepiece. One way to reduce the size of the eyepiece is to construct it out of a block of high refraction-index plastic. This allows the same size enhancement imagery source to be reflected from a smaller and closer spherical mirror section. Such a construction is also very sturdy. Of course, it is also quite a bit heavier than a construction in which only air separates the various optical elements.